1. Field of the Invention
This invention relates to fuel cells, particularly to polymer electrolyte fuel cells.
2. Related Art
A polymer electrolyte fuel cell (hereinafter called PEFC) that uses a polymer electrolyte has merits of high output, long service life, little deterioration by starting and stopping, low operating temperature (approx. 70 to 80° C.) and needs no precise differential pressure control, etc. Therefore, it has a very wide range of applications such as power supplies for electric automobiles, distributed power supplies for industrial or business use and home use and so on.
A unit fuel cell of the PEFC comprises
a membrane-electrode assembly (hereinafter called MEA) having a proton exchange membrane (hereinafter called PEM), which is proton-conductive sandwiched between porous electrodes coated with platinum or alloy catalyst such as platinum-ruthenium alloy and
a unit cell separator having gas flow channels which supply hydrogen gas to the anode and air (oxygen) to the cathode, respectively.
The PEM must be wet to a certain level to let protons move. To keep the PEM wet, a humidified fuel gas is usually fed to the anode or cathode. Various mechanisms have been proposed as apparatus to humidify fuel gases (for example by patent documents 1 to 7 shown below).
A humidifying mechanism using only a water permeable membrane has a problem that hydrogen gas bubbles passing through the water permeable membrane gather in the water supply side and prevent water from passing through the membrane. To solve this problem, a method has been disclosed in patent document 4, which comprises providing a platinum catalyst layer in the water supply side of the membrane, causing the hydrogen gas passing through the membrane to react with the oxygen gas dissolved in the supplied water, and thus eliminating hydrogen bubbles.
Patent document 1: Japanese Application Patent Laid-Open Publication No. 06-124722 (Page 3, FIG. 4)
Patent document 2: Japanese Application Patent Laid-Open Publication No. 07-65845 (Page 2, FIG. 9)
Patent document 3: Japanese Application Patent Laid-Open Publication No. 08-138704 (Page 3, FIG. 2)
Patent document 4: Japanese Application Patent Laid-Open Publication No. 08-138705 (Page 2, FIG. 4)
Patent document 5: Japanese Application Patent Laid-Open Publication No. 08-250130 (Page 4, FIG. 7)
Patent document 6: Japanese Application Patent Laid-Open Publication No. 09-92308 (Page 5-6, FIG. 1)
Patent document 7: Japanese Application Patent Laid-Open Publication No. 11-185777 (Page 3, FIG. 5).
However, a humidifier using a conventional water permeable membrane has a problem that the pressure difference between the fuel gas and the cooling water that passes through the water permeable membrane will make the membrane project towards the low-pressure side during power generation. This membrane projection will narrow the fuel gas flow channels and reduces the flows of the fuel gas and the cooling water.
As a result, auxiliary devices that supply the cooling water and the fuel gas consume more power to supply them against the pressure loss. This reduces the efficiency of the fuel cell system including the auxiliary machines and the fuel cell.